This invention relates to the replication of microstructure-bearing surfaces, and more particularly to a genus of resin-based compositions capable of such replication.
Microstructure replication in resinous surfaces is of importance in such diverse technical fields as the fabrication of traffic signs, in which reflectivity is provided by cube-corner embossed sheeting; the production of Fresnel ophthalmic lens elements and flexible video disks; and the fabrication of “brightness enhancement” or “light management” films (hereinafter sometimes designated “LMF” for brevity) for liquid crystal displays. For replication purposes, it is desirable that the resin have optimum physical properties, including substantial transparency, a glass transition temperature (Tg) high enough for shape retention during storage and use and viscoelastic properties facilitating shaping, typically by molding, and long-term shape retention which includes the microstructure aspects of the shaped articles. Suitable viscoelastic properties include moduli in the glassy and rubbery states within certain ranges, as well as suitable transition temperatures between those states. It is also desirable for the LMF to be adherent to substrates such as polycarbonate and to have good resistance to abrasion and scratching.
Many suitable resinous compositions for the replication of microstructure are disclosed in the patent literature. A patent whose disclosure is generic to a large variety of such compositions is U.S. Pat. No. 4,576,850. The compositions described therein are characterized by “hard” and “soft” segments or moieties in combination with radiation-polymerizable moieties. Most often, all three of these types of segments are present in the same molecule. A key feature of the “hard” segments is the presence therein of cyclic (i.e., carbocyclic or heterocyclic) groups. Later-issued patents frequently make reference to U.S. Pat. No. 4,576,850 for its disclosure of suitable polymeric compositions and precursors therefor.
Optimally, an LMF will have a high refractive index, preferably at least about 1.70. Attainment of a refractive index this high is, for the most part, not possible when using only resinous materials of reasonable cost. The refractive indices of polymers of highly brominated monomers such as pentabromophenyl methacrylate are about 1.71, but such polymers sometimes have unfavorable physical properties.
U.S. Pat. No. 6,291,070 discloses highly transparent nanostructured molded articles having a wide range of refractive indices, prepared from such inorganic materials as titanium dioxide combined with polymerizable or polycondensable surface groups such as those derived from (meth)acrylic and epoxy compounds. There may also be present other monomeric species such as (meth)acrylic acid esters.
U.S. Pat. No. 6,432,526 discloses compositions of high refractive index comprising highly crystalline metallic particles such as those of titanium dioxide, combined with an organic solvent and a dispersing aid to form colloidal particles. Such particles may be combined with organic polymers such as those derived from acrylates and methacrylates to form said compositions.
It is of interest, however, to develop additional high refractive index resinous LMF materials not disclosed in the above-identified patents or in other publications. In particular, it is of interest to provide a means for easily tailoring such materials to various property profiles.